*EDIT* Note that in aggregation expressions null and missing are not equal (i.e $eq of two of them will not return true) so technically this works "correctly" according to aggregation expression semantics, but it's not consistent with $lookup with localField/foreignField which uses find/match equality semantics.

Discovered when debugging expressive lookup not using _id index when looking up based on non-existent field:

{$lookup:{
    from:"coll",as:"x", 
    let:{i:"$foobar"},
    pipeline:[{$match:{$expr:{$eq:["$_id","$$i"]}}}]
}}

Because evaluation of "$foobar" in let doesn't resolve it to a constant expression, the inner pipeline cannot rewrite $expr and doesn't use an index.

Reported on 3.6, tested and reproduced on 4.1.9.